The present invention relates to the processing of gaseous media and more specifically to the treatment of the exhaust gases of internal combustion engines to reduce the emission of pollutants therefrom.
Our earlier patent GB 2 274 412 discloses reactors for the treatment of exhaust emissions from internal, combustion engines to reduce the emission of pollutants such as NOx, carbon monoxide and particulates. The active part of the reactors is a bed of particulate ferroelectric material contained between two gas permeable electrodes across which is applied a potential of the order of tens of kilovolts. In addition to removing particulates by oxidation, especially electric discharge assisted oxidation, there is disclosed the reduction of NOx gases to nitrogen, by the use of pellets adapted to catalyse the NOx reduction.
Somewhat similar systems also are disclosed in U.S. Pat Nos. 3,983,021; 4,954,320 and 5,147,516.
Examples of diesel exhaust particulate filters including gas permeable beds are to be found in European patent application EP 0 010 384; European patents EP 0 244 061; EP 0 112 634; EP 0 132 166 and U.S. Pat. Nos. 4,505,107; 4,485,622; 4,427,418 and 4,276,066.
Problems which occur with those systems which utilise plasmas formed in the exhaust gases as they pass through the bed of particulate material are irregularities in the generation of the plasma due to an uneven distribution of the electric field through the bed of particulate material and arcing or electrical tracking between the electrodes and other parts of the structures of the reactors.
It is an object of the present invention to provide an improved reactor for the treatment of internal combustion engine exhaust emissions.
According to the present invention there is provided a reactor for the plasma processing of gaseous media, comprising a reactor chamber including a gas permeable bed of non-conducting active material contained between at least one gas permeable member adapted to act as an electrode and another gas permeable member and means for constraining a stream of a gaseous medium to be processed to flow through the bed of active material wherein the bed of active material extends beyond the gas permeable electrode or electrodes thereby to isolate electrically the said electrode or electrodes from the means for constraining the exhaust gases to flow through the bed of active material.
Preferably the bed of active material has a structure which is self-supporting. Suitable structures are an open-celled solid foam, a honeycomb, or a fibrous mass. If the bed of active material is not self-supporting, then that part of the bed of active material which extends beyond the said electrode or electrodes is contained within electrically insulating heat resisting members. For example, if the reactor has an axial-flow configuration, then the electrode, or electrodes, may comprise electrically conducting mesh disks with a peripheral annulus of electrically insulating ceramic material. If the reactor has a radial flow configuration, with the electrode, or electrodes being in the form of an electrically conducting mesh cylinder or cylinders then the cylinder or cylinders can terminate in electrically insulating ceramic end pieces.
Preferably a series of gas permeable electrodes is embedded within the bed of active material and the electrodes are so distributed that a desired electric field distribution is created within the bed of active material. In a preferred arrangement, electrodes are arranged in groups of three (or more) and connected to a three (or more)-phase alternating current power supply.